A porous inorganic oxide coating is considered to be a relatively thin coating layer—for example of less than 1 micron thickness—which substantially consists of an inorganic oxide and has certain porosity. Such coatings, like those based on silica, can be used for different purposes, and are increasingly applied to a transparent substrate to reduce the amount of light being reflected from the air-substrate interface, and thus increase the amount of light being transmitted through the substrate. Such coatings can be used as single layer or as part of a multi-layer coating (or coating stack). Typical single layer AR coatings based on thin porous silica layers have been described in e.g. EP0597490, U.S. Pat. Nos. 4,830,879, 5,858,462, EP1181256, WO2007/093339, WO2008/028640, EP1674891, WO2009/030703, WO2009/140482, US2009/0220774, and WO2008/143429.
A single layer AR coating on a transparent substrate typically should have a refractive index between the refractive indices of the substrate and air, in order to reduce the amount of light reflected. For example, in case of a glass with refractive index 1.5 the AR layer typically should have a refractive index of about 1.2-1.3, and ideally of about 1.22. A porous silica (or other inorganic oxide) layer having sufficiently high porosity can provide such a low refractive index and function as AR coating, if its layer thickness is about ¼ of the wavelength of the light; meaning that in the relevant wavelength range of 300-800 nm the thickness preferably is in the range 70-200 nm. This of course means that the size and geometry of pores in such coating should be compatible with said layer thickness.
Such porous inorganic oxide coatings are typically made from a solvent based coating composition comprising inorganic oxide precursors and a pore forming agent. Typically a sol-gel process, also known as chemical solution deposition, is used for making such a (porous) inorganic oxide layer starting from a precursor compound in solution or colloid (or sol) form, for forming an integrated network (or gel) of either discrete particles or network polymers. In such process, the sol gradually evolves to a gel-like diphasic system containing both a liquid and solid phase. Removing remaining liquid (drying) is generally accompanied by shrinkage and densification, and affects final microstructure and porosity. Afterwards, a thermal treatment at elevated temperature is often applied to enhance further condensation reactions (curing) and secure mechanical and structural stability. Typical inorganic oxide precursors are metal alkoxides and metal salts, which can undergo various forms of hydrolysis and condensation reactions. Metal is understood to include silicon within the context of this description.
Such coating composition contains solvent and organic ligands from organo-metallic precursors, which compounds as such will induce some porosity to the inorganic oxide layer. The further presence of a pore forming agent in the coating composition helps in generating suitable porosity in the final AR layer to provide the desired refractive index. Suitable pore forming agents, also called porogens, known from prior art publications include organic compounds, like higher boiling solvents, surfactants and organic polymers, and inorganic particles having sub-micron particle size, i.e. inorganic nano-particles.
Organic compounds and polymers as pore forming agent in such coating compositions may in initial stages after applying the coating to a substrate be present in dissolved, dispersed or other form. After drying the coating, these organics can be removed by known methods; for example by exposing the coating to a solvent for the compound or polymer and extracting it from the coating. Alternatively a compound or polymer can be removed during thermally curing the coating by evaporation, for example at temperatures above the boiling point, or above the decomposition temperature of an organic polymer (i.e. by pyrolysis or calcination). Suitable temperatures are from about 250 to 900° C. A combined treatment of dissolving and degrading/evaporating the compound or polymer may also be applied.
Suitable polymers as pore forming agent can be removed from the coating, and provide a desired pore size of below 200 nm. Examples include organic polymers derived from a.o. styrenic, acrylic and olefinic monomers, including homopolymers and copolymers. In U.S. Pat. No. 4,446,171 use of various organic polymers is described, including PMMA, nitrocellulose, cellulose acetate butyrate, polyvinyl alcohol, and a hydroxyl-functional acrylic copolymer. Polyvinyl acetate is applied in U.S. Pat. No. 5,858,462. In EP0835849, EP1181256 and US20080241373 polyethylene oxide is used as porogen.
Inorganic nano-particles are also used to induce porosity in the coated layer; pores in this case resulting from spaces between non-ideally packed agglomerated particles not being completely filled by the inorganic oxide matrix or binder, as in a.o. U.S. Pat. No. 2,432,484, EP1430001 and W02009/14082. In this last publication a coating solution containing silica nanoparticles of primary particle size 40 nm, an acid having pKa≦3.5, and optionally a coupling agent like tetraethylorthosilicate (TEOS) is used to make a uniform AR coating layer.
Porous, hollow, and core-shell inorganic nano-particles represent a special group of inorganic particles. In US2009/0220774 an AR coating composed of mesoporous silica nano-particles is described, which coating typically comprises pores of diameter 2-10 nm within the mesoporous particles, and pores of diameter 5-200 nm between said particles. The mesoporous silica particles of diameter of 200 nm or less preferably have a porous structure with hexagonally arranged mesopores, and are made with a combination of a cationic and a nonionic surfactant.
WO2008/143429 describes a method of making an AR coating, wherein porous silica particles having particles size of 10-100 nm are produced by a) mixing organic solvent, surfactant and colloidal silica of size 2-50 nm to form silica reversed micelles, b) surface treating the reversed micelles with a silane derivative, and c) removing solvent and surfactant. As surfactant preferably anionic or nonionic surfactants are used. Core-shell inorganic-organic nano-particles are particles with a metal oxide shell and an organic core, which core can be removed—similarly to the organic polymer during curing of a coating as described above—to result in porous or hollow particles embedded in the binder. The organic core can be an organic polymer, like those described above. Such core-shell particles have been described in numerous publications, including U.S. Pat. Nos. 5,100,471, 6,685,966, WO2008028640, WO2008028641, and WO2009030703, and documents cited therein.
Optimum pore size in an AR coating is not only depending on the coating layer thickness as mentioned above, but also on other desired performance characteristics. For example, pore size should not be too large, to minimise light scattering and optimise transparency. On the other hand, if the layer contains very small pores, this may result—under ambient conditions—in non-reversible moisture up-take via capillary condensation; affecting refractive index and making the coating layer more prone to fouling with other components. Such capillary condensation effects have been reported for so-called meso-porous silica, especially having pores in the range 1-20 nm. Too large pores may also deteriorate mechanical strength of the coating, e.g. reduced (pencil) hardness and abrasion resistance. Ideally, pore size can be controlled and selected within a 10-200 nm range to optimize various properties of the AR coating, which is difficult to obtain with prior art systems.
There thus remains a need in industry for a coating composition for making an anti-reflective coating based on a porous inorganic oxide, which provides improved control of pore size and structure, as a tool to improve coating performance in use.
It is therefore an objective of the present invention to provide such an improved coating composition.
The solution to above problem is achieved by providing the coating composition as described herein below and as characterized in the claims. Accordingly, the present invention provides a coating composition for making a porous inorganic oxide coating layer on a substrate, the composition comprising an inorganic oxide precursor as binder, a solvent, and an organic polymer as pore forming agent, wherein the organic polymer comprises a synthetic polyampholyte.